Implantable medical devices, such as pacemakers, defibrillators, cardioverters, and implantable cardioverter-defibrillators (“ICDs”), collectively referred to herein as implantable cardiac stimulating devices, are designed to monitor and stimulate the heart of a patient who suffers from a cardiac arrhythmia. Using leads connected to a patient's heart, these devices typically stimulate the cardiac muscles by delivering electrical pulses in response to measured cardiac events that are indicative of a cardiac arrhythmia. Properly administered therapeutic electrical pulses often successfully reestablish or maintain the heart's regular rhythm.
Implantable cardiac stimulating devices can treat a wide range of cardiac arrhythmias by using a series of adjustable parameters to alter the energy, shape, location, and frequency of the therapeutic pulses. The adjustable parameters are usually defined in a computer program stored in a memory of the implantable device. The program, which is responsible for the operation of the implantable device, can be defined or altered telemetrically by a medical practitioner using an external implantable device programmer.
Programmable pacemakers are generally of two types: (1) single-chamber pacemakers, and (2) dual-chamber pacemakers. In a single-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, a single-chamber of the heart, either the right ventricle or the right atrium. In a dual-chamber pacemaker, the pacemaker provides stimulation pulses to, and senses cardiac activity within, two chambers of the heart, namely both the right atrium and the right ventricle. The left atrium and left ventricle can also be sensed and paced, provided that suitable electrical contacts are effected therewith.
One problem faced with the advent of dual-chamber pacemakers is that when a pacemaker delivers a stimulation pulse to the ventricle during an appropriate portion of a cardiac cycle, this pulse would be sensed by the atrial channel. Therefore, it is a common practice in the art to apply a post-ventricular atrial blanking (PVAB) period upon delivery of a ventricular stimulation pulse, in order to prevent the saturation of the sense amplifiers of the atrial channel. Because ventricular and atrial responses are sensed through the same lead electrodes through which the stimulation pulses are delivered, the resulting polarization signal, also referred to as an “afterpotential”, formed at the electrodes, can corrupt the evoked response which is sensed by the sensing circuits. This undesirable situation occurs often because the polarization signal can be three or more orders of magnitude greater than the evoked response.
Furthermore, the lead polarization signal is not easily characterized; it is a complex function of the lead materials, lead geometry, tissue impedance, stimulation energy and other variables, many of which are continually changing over time. By disabling the atrial sense amplifier, that is applying a refractory or “blanking” period, upon the delivery of a ventricular stimulating pulse, the atrial sense amplifier is not affected by the ventricular stimulation pulse. At a specified time interval after the delivery of a ventricular stimulating pulse, the atrial sense amplifiers are enabled again to sense intrinsic or evoked atrial events.
However, the PVAB period poses a new problem in that it may occur mid-way or even late in the atrial cycle and may therefore result in an atrial channel inability to sense the next intrinsic atrial event. Essentially, the atrial channel is “blinded” to rapid atrial rates precluding proper diagnostic and therapeutic measures by the implanted cardiac device. Instead, a missed atrial event would trigger an atrial stimulation pulse to be inappropriately delivered by the pacemaker. Such inappropriate pacing could endanger the patient by inducing a sequence of events that might induce cardiac arrhythmias.
Another problem faced with the development of dual-chamber pacemakers is that the evoked R-wave (the electrical signal associated with ventricular contraction) subsequent to a ventricular stimulation pulse will typically propagate to the atrium in patients with intact atrioventricular (“AV”) conduction. This propagated signal of a ventricular R-wave in the atria is commonly referred to as a “far-field R-wave” (FFR). Even a premature ventricular contraction (PVC), an arrhythmic event common in many patients requiring implantable cardiac devices, can propagate and produce a far-field signal on the atrial channel. Such far-field signals sensed by the atrial channel could be interpreted as atrial events. This erroneous sensing could easily be misinterpreted by the pacemaker's controlling operations as a change in atrial rate or even an atrial arrhythmia and consequently invoke improper therapeutic measures, potentially harming the patient.
In order to overcome this risk, the post-ventricular atrial blanking period employed upon the delivery of a ventricular pulse is commonly programmed long enough to encompass the far field signal associated with the propagation of a ventricular R-wave subsequent to a ventricular stimulation pulse. This post-ventricular atrial blanking period is commonly programmed to be a fixed time interval, typically 150 msec.
However, this relatively long, fixed post-ventricular atrial blanking period can exacerbate the limitations of a dual-chamber device in that the ability of the pacemaker to detect high atrial rates may be further impaired.
The window of time that the atrial channel is enabled for sensing atrial events is directly reduced as the post-ventricular atrial blanking period is lengthened to eliminate far-field signals from being sensed. Furthermore, conduction time between the ventricle and atrium will vary from patient to patient. In some patients, far field signals associated with ventricular events may occur even later than the typically programmed 150 msec blanking period. Using still longer blanking periods could more severely impair the pacemaker's ability to detect even normal atrial rates.
It would thus be desirable to provide a system and method for automatically adjusting the post ventricular atrial blanking period such that the blanking period following a ventricular stimulation pulse is minimized, thereby allowing the longest atrial sensing window possible in an implantable dual chamber stimulation device. Furthermore, it would be desirable to implement the system and method in a way that allows far-field signals sensed by the atrial channel to be properly interpreted as the ventricular events that they are associated with, thereby excluding them from atrial rate determinations. It would further be desirable to enable the pacemaker to perform this automatic post-ventricular atrial blanking period adjustment without requiring dedicated circuitry and/or special sensors.